The invention relates to and RFI filter and to a method of activating an RFI filter which is connected to a frequency converter.
In frequency converters for speed regulation of electric motors pulse width and pulse amplitude modulation is extensively used when controlling power semiconductors. This modulation has as an unwanted side effect, the generation of conductor transmitted radio noise. EN 55011 describes limit values for transmission of radio noise (Radio Frequency Interference) for units which generate electrical noise. The noise is typically located in the area of some kHz up till about 30 MHz. In order to observe these limits, building in or adding an RFI filter can be done. When designing an RFI filter for a frequency converter it must be taken into account whether the rectifier is controlled or uncontrolled, and also the switching frequency of the inverter must be considered. Further, the length of the motor cables is an essential design parameter. RFI filters can be connected on the output of the frequency converter as well as being placed in the intermediate circuit or on the mains side. This invention concerns RFI filters on the mains side and in the intermediate circuit.
However, the use of RFI filters causes problems if at the same time a fault current circuit breaker is connected to the mains side of the frequency converter. Heavy duty RFI filters have one or more earthed (PE, Protective Earth) capacitors which are so big that the circuit breaker is activated during start up. This problem can be solved by using circuit breakers with a higher trip current or by using circuit breakers with delay circuits so that pulses having a short duration will not cause triggering.
Alternatively the problem with unwanted triggering of the circuit breaker can be solved in designing the RFI filter in a way that reduces the earth leakage current. From EP 0 930 695 an RFI filter is known which makes it possible to connect a frequency converter to a circuit breaker. The three-phased filter comprises in each phase two coils, and between each coil a capacitor is connected to a common capacitor star point. From this star point a parallel connection of a resistor and a capacitor leads to earth.
The disadvantage with the circuit described is the use of relatively large and thus expensive coils. The capacitor C2 limits the earth leakage current but at the same time reduces the performance of the RFI filter. To compensate for this reduction the filter performance has been increased by placing two coils in each phase and it must be assumed, that these coils also have a relatively large inductivity. This large inductivity together with the capacitors leads to a large resonant overvoltage across C2, and in order to limit this overvoltage a resistor R2 is permanently placed parallel to C2.
Further, the circuit will not be applicable to high sensitivity circuit breakers without increasing the size of the coils further.
The RFI filter designer faces another problem, namely the use of symmetrically and non-symmetrically earthed main supplies. In symmetrically earthed mains supplies the star point of the three phases is earthed which to a high degree is the case in Europe. Asymmetrical mains supplies are often used in the United States where one of the three phases is earthed, a so-called delta-earthed mains supply. Frequency converters on asymmetrical mains supplies can cause considerable earth leakage currents, which will more often trigger a circuit breaker than is the case when frequency converters are connected to symmetrical mains supplies. It has been empirically determined that the connection of a frequency converter on asymmetrically earthed mains supplies happens in about 10% of the cases. Based on the above the object of the present invention is to develop a simple RFI filter which can prevent triggering of ordinary (FI interrupters) or high sensitivity earth leakage circuit breakers during start up of a load, especially a frequency converter.
Another object of the present invention is to develop an RFI filter which can be used on ordinary earthed mains supplies as well as on asymmetrically earthed mains supplies.
By using an RFI filter which is connected to a voltage source on the one side and a to a noise generating circuit on the other side, and where the filter seen from the direction of the voltage source comprises a coil connected in series with the supply conductor, and where a branch from the supply conductor incorporates an earthed series connection consisting of a noise-decoupling capacitor and a resistor, the objects of the present invention are met by connecting a switch in parallel to the resistor. The switch can be opened and closed by means of a control signal from a controller.
The advantage with this solution is, that the RFI filter can be used on ordinary earthed mains supplies as well as on asymmetrically earthed mains supplies. On asymmetrically earthed mains supplies the switch is kept open during start up as well as during operation, whereas the switch on ordinary mains supplies preferably is closed during operation. It is, therefore, not necessary for the manufacturer to have two versions of the RFI filter. Instead, the RFI filter can be adapted to the mains supply at the end user.
When operating on ordinary earthed mains supplies it is advantageous to keep the switch open until the noise-decoupling capacitor has been fully charged and first then to short-circuit the resistor. In this way the noise-decoupling capacitor will be directly earthed during the normal time of operationxe2x80x94no series resistor is activexe2x80x94and hereby a reduction in the efficiency of the filter is avoided. The noise-decoupling capacitors and the resistor can thus be sized in a way, that the earth leakage current on activation is smaller than the trigger current for high sensitivity circuit breakers. In addition, the present invention has the added advantage that the size of the coil can be kept relatively small.
If the RFI filter is used on polyphased mains suppliesxe2x80x94typically three-phasedxe2x80x94the resistor can simply be connected to the star point of the noise-decoupling capacitors, and the switch connected in parallel from the star point to earth.
It is especially advantageous if the resistor together with the relay and a further capacitor form a parallel connection between the star point of the noise-decoupling capacitors and earth. In this way the additional capacitor during operation on asymmetrically earthed mains supplies improves the filter performance.
The filter according to the invention can also be used in d.c. circuits such as the intermediate circuit of a frequency converter. In connecting the noise-decoupling capacitor to the positive- or negative side of the intermediate circuit the circuit breaker can be protected against unwanted triggering.
If the noise-decoupling capacitor is connected midway between two series connected capacitors, which are placed between the positive and the negative bus bars of the intermediate circuit, the noise-decoupling capacitor can be protected against high voltage. This means that a lower rated and thus inexpensive component can be employed.
To enable the use of the RFI filter also on asymmetrically earthed mains supplies a capacitor is connected in the intermediate circuit, where one pole is connected to the positive or the negative bus bar and the second pole is connected to earth.
In order to obtain the optimum attenuation of radio noise transmitted by cable, the RFI filter according to the invention should be placed on the mains side as well as in the intermediate circuit. This gives a two-section filter. It is particularly advantageous if the switches are activated at the same time by the same controller.
Preferably, the described switches are relays, however, transistors can also be used.
The filter is particularly applicable in a frequency converter for speed control of a motor where a circuit breaker is placed on the mains side of the frequency converter.
Further, the invention relates to a method for activating an RFI filter which is connected to a frequency converter, where the RFI filter comprises an earthed series connection and where the series connection comprises a noise-decoupling capacitor in series with a resistor. In particular it is advantageous if a switch is placed in parallel across the resistor and kept open until the noise-decoupling capacitor is essentially fully charged, whereafter the switch is closed via a control signal from a controller thus bypassing the resistor. This method prevents unwanted activation of the earth leakage circuit breaker during start up.
In using two RFI filters placed respectively on the mains side and in the intermediate circuit of the frequency converter, and in using one controller to close the switches simultaneously in the two filters, the circuit is simplified and the total attenuation of the noise is increased.